The field of the invention is generally that of toilets and, more particularly, that type of toilet which has a flush tank positioned above the level of a toilet bowl and which is adapted to normally be refilled after each flushing operation to a predetermined level and to then be abruptly released during a flushing operation, in response to the operation of a flush lever or handle, so that the entire quantity of water in the flush tank is then abruptly discharged, under the influence of gravity through a large discharge valve and into the toilet bowl for effectively flushing it out. This prior art type of toilet discharges a very large quantity of water during each such flushing operation, and in the light of ever-increasing awareness of the necessity of conserving water, particularly in certain relatively dry or drought-stricken areas, it would be a vast improvement if a smaller quantity (preferably selectively adjustable) of water could be discharged during each flushing operation. The conventional toilet flush tank defines an interior chamber of substantial volume, and there are large numbers of such relatively expensive pieces of equipment already in the field, which would be very expensive to replace with flush tanks of smaller volumetric capacity. The desirable thing would be to provide some way of making it possible to reduce the volume of water discharged during each flushing operation while using the pre-existing, relatively large flush tanks which are already installed in millions of homes and buildings. While means for controlling the main discharge valve might accomplish this, all such arrangements which have been developed to date are arrangements which partition off portions of a tank so that only a part of the tank will be emptied during a flushing operation, and they require relatively complicated and expensive equipment and have certain operational disadvantages, including installation problems, adaptability problems with respect to the installation of same in various different sizes and types of pre-existing flush tanks, and the maintenance problems. It is clear that it would be highly desirable to provide a simple, virtually maintenance-free and relatively inexpensive means for providing for a reduced volume (in certain forms, selectively adjustably reduced volume) of water discharged during a flushing operation, which would have none of the above-mentioned prior art disadvantages and yet would be efficient, effective and adaptable to a great variety of types and sizes of pre-existing flush tanks. It is precisely such a highly desirable and advantageous type of simple, inexpensive, easy-to-install and virtually universally adaptable device for use with either newly manufactured flush tanks or with the vast number of pre-existing flush tanks, which have been manufactured and are now in use, which is provided by and in the present invention, and which has advantages completely overcoming the above-mentioned problems, disadvantages and limitations of prior art constructions intended for the same purpose, and all of which advantages flow from and occur by reason of the specific features of the invention pointed out hereinafter.